A simple workaround to the problem of how to remove all the songs from a Spotify play queue is to create a special Playlist with a very very very short track in it. All that you need to do is to double-click it at any time and your main cache will be cleared.
PLEASE NOTE that these workaround instructions relate to using Spotify on a PC. We've no idea whether this will also work on a tablet, iPad, iPod, etc.
Here's what to do:
(1) with the Spotify program open, choose File then New Playlist.
(2) you'll see a new playlist appear in the left-hand sidebar which is waiting for you to give it a name. Type in Clear Queue (or anything you like! ... but 'Clear Queue' is more helpful), hit Return and you now have an empty Spotify playlist location called 'Clear Queue' somewhere in your left-hand sidebar.
(3) next, type 'kyuss yeah' into the search box (without the apostrophes) at the top of the page and hit Return. Spotify will show you a track called 'Yeah' by an Artist called 'Kyuss'. The importance of this track does not relate to the artist or the title .... it's important because it's only 4 seconds long!
(4) Right-click the track and choose 'Add to...' then 'Clear Queue'. The 4 second long track has now been added to that playlist.
(5) That's it. To clear the Spotify play queue at any time just double-click the 'Clear Queue' playlist .... the 4 second track will play ..... "YEAH!" ..... and your playlist cache is clear!
(6) You can of course put whatever song or track you like in the new playlist but by choosing one that's very very short means that you won't have to keep stopping the song every time you've cleared the Spotify play queue.